Several techniques have been described and used to distribute hydrophobic compounds, particularly non-polymeric compounds, in polymeric latex particles. For example, U.S. Pat. No. 4,368,258 (issued Jan. 11, 1983 to Fujiwhara et al) describes various techniques for incorporating dye-forming compounds and ultraviolet light absorbers into latex particles for use in photographic materials. Generally, the techniques all involve mixing a hydrophobe in solid state, a water-miscible organic solvent and a polymeric latex. The latex purposely contains large amounts of dispersant (also known as a surfactant or emulsifier).
U.S. Pat. No. 4,199,363 (issued Apr. 22, 1980 to Chen) describes a process for "loading" hydrophobes into latex particles whereby the compounds are dissolved within a water-miscible organic solvent. The process generally comprises gradually adding a "loadable" latex (one designed to receive the hydrophobe) to an organic solvent solution of the hydrophobe in such a manner that the hydrophobe is distributed with the latex particles predominantly. The resulting loaded latex is used in various photographic materials. The loadable latex further contains a surfactant for minimizing coagulation of latex particles.
While U.S. Pat. No. 4,199,363 provides useful materials for use in various photographic materials, there is a need for incorporating hydrophobes in polymeric latex particles to which can be attached biological compounds for various biological processes and assays. For example, it would be useful to have insoluble dyed reagents for use in various assays, such as agglutination assays, for the determination of a ligand of some type. These reagents could include a latex containing dyed particles to which a receptor has been attached. A receptor is a biological or chemical compound which specifically reacts or binds to a ligand of interest. Examples of ligand and receptor include avidin and biotin, and antibodies against corresponding antigens.
In preparing such biological reagents, it has been found that the process described in U.S. Pat. No. 4,199,363 has serious drawbacks. The described process whereby loadable latex is gradually added to the hydrophobe solution is prone to coagulation if the addition process is not carefully controlled. However, the likelihood of coagulation can be minimized with the use of surfactants which protect the latex particles.
However, in the preparation of insoluble biological reagents containing a hydrophobe and a biological compound attached to the particles, the presence of surfactant in the latex adversely affects the activity of many biological compounds. For example, the presence of surfactant often interferes with the reaction of antibody with its corresponding antigen. Such interference inhibits accurate and sensitive detection of various ligands in important diagnostic and analytical procedures. Yet, the presence of a surfactant appears to be essential in the practice of the method of adding hydrophobe to latex particles which is described in U.S. Pat. No. 4,199,363.
It would be desirable to be able to incorporate hydrophobes into latex particles for use in the preparation of biological reagents without the use of surfactants.